1. Field of the Invention
This invention relates to improvements in integrated circuit structures using self-aligned contacts to the base of an active device. More particularly, this invention relates to an integrated circuit device having self-aligned local oxide to isolate the extrinsic base from another electrode of the transistor, such as an emitter.
2. Description of the Prior Art
In the construction of a high performance bipolar transistor using a self-aligned contact to the base region, an extrinsic base is formed in the silicon substrate adjacent the intrinsic base of the device. Such a construction is described in Jambotkar U.S. Pat. No. 4,319,932 and in an article by Ning et al entitled "Self-aligned Bipolar Transistors for High-Performance and Low-Power-Delay VLSI", published in the IEEE Transactions on Electron Devices, Vol. ED-28, No. 9, Sept. 1981, on pages 1010-1013. However, the formation of an emitter in or above the intrinsic base in this type of structure can result in the formation of a parasitic P-N junction between the emitter and the extrinsic base. This parasitic diode degrades the current gain or beta of the transistor.
FIG. 1 shows a typical prior art construction of such a device. A polysilicon layer is normally anisotropically etched to expose a portion of the silicon substrate in which the intrinsic base is formed. An extrinsic base region is then formed in the substrate adjacent the intrinsic base by diffusing dopant through the polysilicon and into the silicon substrate, usually at the same time as oxide is formed along the polysilicon sidewall. Contact alignment between the polysilicon layer and the extrinsic base region is thus assured.
When an emitter is subsequently formed on or in the surface of the intrinsic base, the amount of separation of the emitter from the extrinsic base will depend upon the thickness of the oxide on the sidewall of the etched polysilicon layer and the amount of lateral diffusion of the extrinsic base. Thus, the current gain of the transistor becomes sensitive to the lateral thickness of the oxide wall layer which separates the extrinsic base from the emitter.
It would, therefore, be desirable to independently isolate the extrinsic base of a self aligned contact integrated circuit device from the emitter portion of the device independent of the thickness of the wall oxide.